A digital data receiver often includes an analog filtering section that conditions an input signal after it has been down-converted to a baseband or intermediate frequency. The analog filtering section removes noise and unwanted frequency components from the down-converted signal to prepare the signal for digitizing. In a conventional digital receiver, the filtering circuit has a fixed bandwidth that is set to accommodate the anticipated baud rate of the incoming signal and to optimize the signal quality and the quality of the decoded data.
Signal quality is adversely affected by both intersymbol interference (ISI) and adjacent channel interference (ACI). Analog filtering circuits may be implemented to reduce ISI, ACI, or other electronic noise associated with digital signal transmissions. For example, ISI is reduced when the filter bandwidth is widened and ACI is reduced when the bandwidth is narrowed. Unfortunately, conventional fixed bandwidth filters inherently increase the amount of ISI when they are tuned to reduce ACI, and vice versa. As such, conventional analog filtering circuits in digital receivers are usually tuned to a less-than-optimum bandwidth with respect to ISI and ACI, which are often a priori unknown.
It may be desirable to receive digital data conveyed over a broad range of baud rates. However, conventional digital data receivers that utilize filtering circuits with fixed bandwidths may only have a narrow operating range, e.g., a percentage of the minimum operating baud rate. Outside of this narrow range, such conventional filters may not be tuned to sufficiently reduce ISI and/or ACI. Thus, fixed bandwidth filters may be undesirable for applications that operate over a baud rate range of more than several MHz.
The bandwidth accuracy of conventional tunable analog filters is only about 10%. Although such accuracy may be sufficient to enable a digital receiver to gain symbol synchronization, the bandwidth inaccuracy may produce an unacceptable bit error rate (BER) resulting from excessive ISI or ACI. To minimize the BER in some applications, it may be necessary to maintain bandwidth accuracy to within 5% or less. Unfortunately, conventional fixed bandwidth filters are not responsive to fluctuations in BER, ISI, or ACI.
In addition, the bandwidth of analog filters drifts in response to changes in operating temperatures, variations in assembly techniques, the age of the electronic components, and component tolerances. The bandwidth stability of analog filter circuits may be improved by utilizing expensive components with tight tolerances or by maintaining strict manufacturing guidelines. Moreover, such filters often must be individually aligned to meet the desired receiver specifications. Thus, procurement, manufacturing, and maintenance costs are excessive when compared to less precise, easily manufacturable, non-aligned filtering circuits.